[Mike51Merc]

Manifold (below the throttle plates) vacuum drops when the throttle is opened. Venturi (above the throttle plates) vacuum increases with engine RPMS. They are kind-of opposite but not exactly.

A "normal" or "typical" vacuum advance distributor increases timing advance when the vacuum signal drops.
A load-a-matic advance increases advance when the vacuum signal increases.

So, no, you can't T them together and expect good results.